1. Field of Invention
This invention relates to seismometers, specifically a seismometer based upon magnetic suspension where the sensitivity associated with each principal axis is adutable.
2. Description of Prior Art
A seismometer is a device that is used to record seismic activities. Conventional seismometers have a mass suspended from a spring. On the end of the mass is a device that records the seismic activity. These seismometers create their own natural frequency and are limited by mechanical friction. Expensive electronic devices can correct this, however, a seismometer utilizing magnetic suspension would not have these problems and furthermore; higher sensitivities may be achieved with this technique which is described in patent U.S. Pat No. 3,626,364 issued to Simon et al. However; the magnetic suspension seismometer invented by Simon et al utilizes three independent seismic masses to detect a seismic activity, in all three axis. Furthermore, the feedback mechanism utilized by Simon et al is strictly proportional and hence not capable of completely replacing any desired mass spring combination. In addition, Simon et al constructed three scalar magnetic fields, while the present invention constructs a single four-vector magnetic field capable of measuring seismic activity with only a single seismic mass in all three axis.
The construction of present invention also naturally lends itself to measurement of gravitational perturbation. Most Prior gravimeters are based on mass and spring combination as described in U.S. Pat No. 4,497,205, issued to Hans Scheurenbrand, Stetten, Fed. These type of gravimeters have a limited range for sensitivity adjustment. This is primarily due to the filet that the stiffness adjustment on the springs are very cumbersome and limited. Other gravimeters use a mass and a restoring force other than spring, such as an arrangement of permanent magnets as described in the German Pat No. 1,230,577. Again such an arrangement will have a very limited range for sensitivity adjustment, due to the fact that the magnetic field of the permanent magnet is predetermined. The present invention utilizes adjustable vector magnetic field as the restoring force mechanism. With this arrangement, a set of virtual springs are realized, that essentially act as springs, but the major advantage is the adjustability of their stiffness and hence the sensitivity of gravimeter may easily be adjusted. Another advantage of the present invention is that the physical spring used in conventional gravimeters change their stiffness over a period of time and requires compensation, with the present invention there is no periodic adjustments.
The present invention being sensitive to gravitational perturbation, may be utilized to measure exoatmospheric attitude not measurable by conventional ultrasonic echo ranging techniques as described by U.S. Pat No. 4,979,154. Since the gravitational force decreases as a function of altitude the current in the electromagnets in the present invention will decease in a predictable manner in higher altitudes. Hence, the total current will be indicative of altitude. The present invention is also sensitive to pitch and roll motion, This is due to the fact that four-vector magnetic field changes its shapes when the structure is tilted to maintain the position of seismic mass, and hence the required current distribution changes providing means to measure angular displacement.